W
Winfield Hill
Guest
Jonathan Kirwan wrote...
that current is voltage divided by the resistance, so current-source noise
density is given by ac voltage-noise density divided by the ac resistance,
namely i_n = (e_n + sqrt(4kTR)) / (R + r_e). Manipulating equations on AoE
page 436, we see that e_n = (4kT r_e/2)^1/2 (ignoring r_bb). So if the dc
voltage across the emitter resistor, Ie*R, is greater than 50 to 100mV, so
I*R >> kT/qI, then the current-source noise density is largely determined
by the bias resistor's Johnson noise density, i_n = sgrt(4kT/R), and not
the transistor's shot noise. This can be used to create a nearly-perfect
quiet current source, using a moderate to high bias voltage (even 100 to
500V), regulated from a modestly-quiet voltage source.
--
Thanks,
- Win
(email: use hill_at_rowland-dotties-org for now)
It's simple enough, but a rather powerful result nonetheless. Ohms law saysChristian Rausch wrote:
At the bottom of the first column of p.432, H&H mention that the standard
transistor current source runs quieter than shot-noise-limited. Anybody
out there who knows a little more (literature, math) about this?
Well, perhaps Win can answer what he meant here (more likely if you ask in
sci.electronics.design, I think.) But I'd imagine as a hobbyist-guess that
the "independence" mentioned earlier isn't entirely true for this case --
some of the current is dependent in some way or the probability of emission
is high, so the integral over all the behavior is no longer quite Poisson.
that current is voltage divided by the resistance, so current-source noise
density is given by ac voltage-noise density divided by the ac resistance,
namely i_n = (e_n + sqrt(4kTR)) / (R + r_e). Manipulating equations on AoE
page 436, we see that e_n = (4kT r_e/2)^1/2 (ignoring r_bb). So if the dc
voltage across the emitter resistor, Ie*R, is greater than 50 to 100mV, so
I*R >> kT/qI, then the current-source noise density is largely determined
by the bias resistor's Johnson noise density, i_n = sgrt(4kT/R), and not
the transistor's shot noise. This can be used to create a nearly-perfect
quiet current source, using a moderate to high bias voltage (even 100 to
500V), regulated from a modestly-quiet voltage source.
--
Thanks,
- Win
(email: use hill_at_rowland-dotties-org for now)